1. Field Of The Invention
This invention relates to the field of magnetic recording, and in particular to the duplication of magnetically recorded information by contact transfer from a magnetically recorded master medium to a slave magnetic medium in the absence of a transfer magnetic field.
2. Description Relative To The Prior art
The expanding use of magnetic recording has generated an extensive need for low cost, high quality duplicate magnetic copies of original recordings. Some applications, such as tape copies for playback on conventional audio reproducing machines, require a linear transformation of the signal from the master to the slave copy, while other applications, such as the duplication of non-analog i.e., digital, servo or format information, or f.m. recorded videotapes, allow the use of non-linear transfer methods. Both the linear analog and the non-linear digital applications are generally performed by playing back the master on an appropriate reproducing system which drives a plurality of slave recording systems. For the linear transfer requirements, anhysteretic recording techniques known in the art are used, while for the non-linear applications hysteretic recording is acceptable.
In either case, an essential characteristic of the slave copy is that it be of high enough coercivity to resist demagnetization in normally encountered environments. Coercivities of 300-500 Oersteds are commonly employed, although a coercivity closer to 1000 Oersteds is necessary to protect the copy against accidental erasure if the slave experiences exceptionally high magnetic fields. Products incorporating the slave copy may be subjected to such high fields if the products are sold in an environment protected by a magnetically activated anti-shoplifting system currently employed in numerous retail enterprises.
Besides machine to machine copying, contact duplication methods are known in which the master medium and the slave medium are placed in intimate contact and transported through an alternating magnetic field, called the bias field. The coercivity of the master medium is always much greater than the coercivity of the slave, and the bias field strength is greater than the coercivity of the slave and less than that of the master. As the media pass through the alternating bias field, the magnetic moments of the particles in the slave are switched back and forth. As the bias field is reduced through the coercivity value of the slave, the slave particles come to rest magnetized parallel to the field of the master. This effects the magnetic transfer of the recorded information from the master to the copy. The bias field switching of the magnetization in the slave does not substantially affect the source recording on the higher coercivity master. It will be noted that this technique is effectively the same as that practiced in anhysteretic recording in the magnetic recording art.
U.S. Pat. No. 2,738,383 entitled "Method and Apparatus for Duplicating magnetic Recordings and Tape Record Members" discloses the use of a transfer field to effect master to slave contact duplication. U.S. Pat. No. 3,472,971 entitled "Magnetic Tape Duplicating Device With Fluid Pressure Applied Through Head Gap" discloses apparatus for ensuring intimate contact between the master and slave while applying a transfer field to the media during duplication.